The present invention relates generally to the field of protective devices for electrical loads and circuitry, and more particularly to a technique for extending a range of overload protection in a device serving loads of substantially different rating.
In the field of electrical devices and circuits, many arrangements have been proposed and are currently in use for providing power and for protection of loads. For example, for motor protection applications, protective circuitry typically includes fuses, circuit breakers, thermal overload protective devices, and so forth. In a typical application, the protective circuitry is specifically adapted for the size of the load, that is, for its current rating. Motor circuitry, for example, may be protected by reference to the motor full load current rating, with overload protection being provided by reference to a multiple of the full load current rating, and instantaneous tripping being provided by a specifically-selected electromagnetic device, such as a circuit breaker. The circuit breaker provides for a higher current tripping level, although faster tripping, with a gap in currents being provided between the trip current of the thermal overload device and the instantaneous trip device.
While arrangements such as these afford adequate protection of motors, they are not without drawbacks. For example, specific components and circuits are typically designed and selected for each type and rating of load. The resulting arrangements require a number of separate components of different ratings, assembled in a large number of combinations. Elevated manufacturing, stocking, and associated costs may thus result, particularly where a user has many different motors of different ratings, or where a supplier provides protective circuitry to many different users having a range of motor products.
It would be advantageous, therefore, to provide protective circuitry that can be employed with a broader range of loads, while providing adequate protection from both thermal overload and high currents that would normally require an instantaneous-type trip. Significant challenges exist, however, with such devices in view of the current approach of selecting the instantaneous trip device based upon the particular load being serviced.